Theories of electrokinetic phenomena on a soft surface, i.e., a hard surface covered with an ion-penetrable polyelectrolyte layer are reviewed. Electrokinetic flow between two parallel plates with soft surfaces is analyzed to derive analytic expressions for various electrokinetic quantities such as electro-osmotic velocity and streaming potential. These phenomena are characterized by two parameters, that is, the density of the fixed charges distributed in the surface layer and the softness parameter. An Onsager relation is found to hold between electro-osmosis and streaming potential. The physical meaning of the softness parameter is elucidated.

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This paper aims to show aspects of complex formation in the polyelectrolyte nanogel system. Two sorts of systems, a polyanion (PA) and a cationic nanogel (CG) as well as an anionic nanogel (AG) and CG, were employed and studied mainly in the absence of simple salts by a combination of dynamic and static light scattering techniques. The main results are summarized as : (i) An intra-particle CG complex of PA forms via charge neutralization which follows a 1 : 1 stoichiometry; but (ii) the bound PA dissociates from the complex upon addition of CG, polycation or KCl, meaning that the polyelectrolyte exchange reaction takes place. (iii) The complexation upon addition of AG to CG whose charge number is grater than that of AG follows 'random model' in which the added AG uniformly binds to all of CG via a strong electrostatic attraction, whereas (iv) the addition of CG to AG results in a complex according to' all-or-none model' in which a part of AG preferably binds to the added CG to neutralize its charges, but the other part is uncomplexed and remaining in the system. These can be understood by considering that the nanogels consist of cross-linked polyions and have the obscure surface due to dangling chains.

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Sedimentation and electrophoresis of porous materials; colloidal floc and colloidal particles covered with adsorbing polyelectrolytes, are visited to examine the characteristic length scale of transport phenomena. In the sedimentation, the size of floc is dominative in the determination of Stokes drag, while the permeability is determined by the large pore in flocs. However, if the floc is placed in the electric field, outside fluid can easily penetrate into the floc by means of electric osmosis. In this regime, the characteristic length scale of hydrodynamic drag is Debye length. In the case of electrophoresis of latex particle covered with adsorbing polyelectrolyte, the characteristic length scale of permeability was estimated same as the distance between charged segments.

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Humic substances are natural polyelectrolytes that play an important role in the speciation of metal ions in soil and aquatic environments. In this paper electrostatic models developed for humic substances are reviewed and their validity is examined by comparing the model predictions with the results of the fluorescence quenching method. For humic acid, a kind of humic substances, the models, where the electrical double layer around a humic acid molecule is explicitly considered, are in good agreement with the experiments. On the other hand, the Donnan models tend to overestimate the negative potential of humic acid.

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Bacteria in natural environments are mostly in an attached state. The surface characteristics of bacterial cells greatly affect cell attachment and hence bacterial ecology. Under normal conditions, bacterial cell surfaces are negatively charged. It was revealed that slower growing strains are less negatively charged and more hydrophobic at their cell surfaces. Bacterial cells carry various polymers on their surfaces. The polymer layers on bacterial cells reduce surface electric potential, resulting in a low energy barrier between bacterial cells and substratum surface of both negatively charged. Some experimental results for cell attachment force are shown as well as the effect of interfaces on bacterial activities.

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Kinetic analysis of intraparticle mass transfer of a solute is generally performed for a large number of particles. However, since the intraparticle mass transfer depends on the particle size, particle-particle distance, and so forth, measurements for single microparticles are indispensable for the quantitative analysis. Thus, microspectroscopy combined with microcapillary injection and manipulation technique has been developed, and the intraparticle mass transfer has been investigated on the basis of the direct measurements of extraction and release rates of a solute for single porous microparticles. In this review, the principles of the microcapillary manipulation-microspectroscopy technique and the diffusion of an organic solute in nanometer-sized pores of single ODS-silica gel and silica gel microparticles are described. The diffusion in the nanometer-sized pores is discussed in terms of pore and surface diffusion. slow desorption from the pore walls. and so on.

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